The completion rate on deep passes in this game is unrealistically good, while the completion rate on short passes is unrealistically bad, even with custom sliders. Therefore the deep pass is by far the biggest threat you face - and your biggest offensive weapon - in HC09, and pass defense is paramount.
There are two basic ways to defend the receiver: zone or man schemes. There are also two basic ways to defend the passer: blitz or coverage. Put'em together and you've got four basic combinations, and examples of some teams that I believe practice them as of career starts: man coverage with safety help (Falcons, Packers), man coverage with blitzes (Cowboys, Titans), zone-blitz schemes (Seahawks, Steelers), and zone coverage without blitzing (Jaguars, Texans, all the Tampa-2 teams: CHI, TB, DET, MIN).
The Giants and Eagles default AI runs combo-coverage blitzes, which are both the hardest and easiest things to defeat, and I'll talk about them separately.
None of the four schemes works against all offenses all the time, no matter how good your players and coaches are. If you call plays or supersim, you
must learn to recognize what sorts of passes your opponent calls, and you must use the right scheme(s) against the AIs favorite plays.
For example, I love to play press-man coverage with blitzing; after all, the least productive passer is the one eating turf with the unthrown ball in his hands. When I look at the scouting report for your opponent, I look for his favorite pass plays (% called): does he operate from shotgun much? Do his backs and tight-ends pass-protect much, or are they out on routes? Does he do much play-action? If your opponent operates from under center, doesn't protect with his backs, and shows few if any play-action passes, it'll be a career day for your blitzers.
On the other hand, the offenses of Ophamer's Patriots, Dungy's Colts, Coughlin's Giants and Gruden's Bucs are happiest and most productive when I blitz, either zone or man, and are least productive when I use seven- or eight-man zone coverage. All three AIs love shotgun formation and have plays designed to routinely foil man coverage. Routinely blitzing those teams is a recipe for losing games. So I adapt my defensive play-calling to account for what teams want to do.
You will also find some teams' AI playcaller return to (what it thinks is) a "money play" over and over again, especially in the fourth quarter when it's losing. If you observe that play in action and can find a defense to counter it, you'll get sack after sack, or interception after interception.
Likewise,
only some of your pass plays will work against any given opponent, and you
must learn to recognize what scheme the opposing defense runs and how to attack it.
Against zone coverage, you need to find route combinations that exploit the voids in the zones - you need to hit'em where they ain't. A team that runs Cover-3 or Cover-4 zone schemes, with or without blitzing, is going to be difficult to beat with the deep ball, but crossing routes in the 5-10 yard vicinity will generally work. Versus a Tampa-2 team you can chuck it deep if you find a vulnerable safety; you can use route combinations like flats-and-curls; or you can use five-yard routes like TE hitches and do ball-control between the D-line and linebackers. I wrote up a thread on attacking the Tampa-2
a while back; there's a few more tips there.
Against a man squad that rarely blitzes, you can run almost anything, but double-moves and play-action crossing patterns are your best bets for significant chunks of yardage. West Coast passing style was designed to defeat deep-safety man coverages, so slants and other short patterns are easy yards. Against man-blitzes, adjust your pre-snap pass protection carefully for standard passes and/or use play-action bootlegs all day, every day.